There were plenty of reasons the Giants’ pass rush was not going to hit the ground running this season and immediately put a scare into opposing quarterbacks. There was a new scheme, rust on holdout Michael Strahan, and a knee problem for Osi Umenyiora after six snaps.

Sure enough, the front four got off to a slow start in the first two games, and one of the prominent sack-masters wasn’t the least bit surprised.

“It takes a while; we’ve never, ever since I’ve been here, started fast as far as pass rushing goes,” Umenyiora said. “I don’t know why. We’d like for it to change but it just never has. I think it turned the corner last week.”

Though Umenyiora and Strahan as starting defensive ends have not caused any havoc (neither has a sack in the first three games), there are signs galore that the front four is making real progress. Umenyiora and Strahan last week each did a better job applying pressure off the edges, allowing for singled-up blocking inside, which benefits the tackles.

In last week’s 24-17 comeback victory in Washington, linebacker Mathias Kiwanuka was credited with 1½ sacks, rushing as a down lineman in the tackle spot, and Justin Tuck had his hand in one-half sack, also when he moved inside to tackle.

Tomorrow night against the Eagles, with Donovan McNabb coming off a monster game but still somewhat less mobile than usual coming off last year’s knee surgery, it is imperative for the Giants to show their defensive improvement was not a blip on the radar screen. The Eagles are coming off a 56-point outburst, and McNabb is coming off a four-touchdown performance that statistically calculated to a perfect quarterback rating of 158.3.

Strahan’s next (and first) sack lifts him out of a tie with Lawrence Taylor and thrusts him atop the franchise list. Of Strahan’s 132 ½ sacks, he has dropped McNabb 11½ times, more than any other quarterback. McNabb last week against the Lions removed the bulky knee brace he wore in the first two games and, according to Giants linebacker Antonio Pierce, “didn’t look as stiff.”

The Giants must hope McNabb is not as effective as he’s been the past four times he’s faced the Giants, when he tossed nine touchdown passes and no interceptions.

Dealing with McNabb, even coming off the serious knee injury, is a different challenge than the one the Giants faced last week when they subdued Jason Campbell, who was making his 10th NFL start. Years ago, the issue with McNabb was preventing him from escaping the pocket; he used to be a devastating scrambler, he’s certainly not that any more.

“You got to hit him,” Strahan said. “No quarterback likes to be hit and I think in Donovan’s case coming off the knee we just got to put pressure on him, get him to run out of that pocket and move around a little bit and make him uncomfortable. It’s not easy to do, because they have a line that’s very talented and have played together for a lot of years.”

That’s where the pass rush comes in. Last week, the defensive line got humming only after defensive coordinator Steve Spagnuolo inserted two natural ends (Tuck and Kiwanuka) into the tackle positions in obvious passing situations. Along with Strahan and Umenyiora, it’s not a foursome the Giants want to line up on run-stopping downs, but it does present a look that is difficult keep out of the backfield.

“You have four outstanding pass rushers on the field,” Umenyiora said.

The offensive guards tend to run out to help with Strahan and Umenyiora but can’t do that nearly as often with Kiwanuka and Tuck lined up at tackle.

“They’re not really able to do that any more,” Umenyiora said. “It gives everybody a good advantage.”